Natural cheese is generally made by developing acidity in milk and setting the milk with a clotting agent, such as rennet, or by developing acidity to the isoelectric point of the protein. The set milk is cut and whey is separated from the resulting curd. The curd may be pressed to provide a cheese block. Curing typically takes place over a period of time under controlled conditions.
Products having flavor and body characteristics similar to those of natural cheese have been made by replacing at least a portion of the animal fat of the milk with a vegetable fat such as corn oil, and/or by replacing at least a portion of the casein of the milk with an analog protein.
It is well known to provide a product having some of the characteristics of natural cheese by grinding a natural cheese, and heating it with an emulsifying salt. The name given to the resulting product depends upon the ingredients used and its composition and, in some instances, is determined by regulations promulgated by the U.S. Food and Drug Administration 21 C.F.R. .sctn.133.169-180. For example, the term "pasteurized process cheese" refers to a product comprising a blend of cheeses to which an emulsifying agent, usually an emulsifying salt, and possibly acids, have been added, and which has then been worked and heated into a homogeneous plastic mass.
The term "pasteurized process cheese food" refers to a product which is prepared from the same materials and the same processes used for manufacture of process cheese. However, cheese food generally has dairy ingredients added thereto, such as cream, milk, skimmed milk, whey, or any of these from which part of the water has been removed (e.g., concentrated skimmed milk). The moisture level in process cheese food is generally higher than that of process cheese and may be up to about 44%. Fat is generally present at a level of not less than 23%.
The term "pasteurized process cheese spread" refers to a product which is similar to cheese food, in the sense that it can contain the indicated dairy ingredients. Process cheese spread, however, may have a moisture level as high as 60%, and a minimum fat level of 20%.
Process cheese, process cheese food and process cheese spread are referred to as "standardized products", since their methods of manufacture and composition are determined by Federal Standards of Identity.
As used herein, the term "process cheese-type products" includes those products known and referred to as "pasteurized process cheese", "pasteurized process cheese food", "pasteurized process cheese spread", and "pasteurized process cheese product". "Process cheese type-products" also includes products resembling process cheese, process cheese food, process cheese spread and process cheese product, but which may not meet the U.S. Federal Standards of Identity for any of the above products in that they may contain ingredients not specified by such Standards, such as vegetable oil or vegetable protein, or may not meet the compositional requirements of such Standards. Process cheese-type products also include products having flavor and texture similar to those of a process cheese-type product regardless of the ingredients or manufacturing steps employed, and regardless of whether the Standards have been met.
In prior manufacture of process cheese-type products, two forms of batch cookers have been generally known, one being a steam jacketed kettle equipped with a mechanical agitator and the other being a horizontally extending cooker also known as a "lay-down" cooker having a screw in the cooker chamber. Live steam is injected directly into the chamber and the raw material at atmospheric pressure as the raw material is agitated by the screw. The raw material is heated in the cooker to a temperature of at least 150.degree. F., usually about 160.degree. F. to 190.degree. F., and is held at that temperature for at least 30 seconds, and usually for about 5 minutes, depending in part upon the raw material and the desired cheese product.
The conventional cheese cookers used in the manufacture of process cheese products are essentially batch-type cookers in which an amount of raw material is introduced into the cooker, and heated and agitated to provide a homogenized molten mass. The molten mass is then intermittently discharged from the cooker for appropriate handling.
The design and operation of the cooker is critical in obtaining a good process cheese product. Mere heating of the cheese blend without proper agitation results in separation of the fat, thus failing to provide a satisfactory emulsion. Too much agitation during heating results in over-emulsification and a process cheese product having undesirable body characteristics. Also, maintenance of the process cheese at the cooking temperatures for an extended period of time is undesirable and burn-on of the cheese onto the sides of cookers has been a problem.
When the cheese has been cooked to the degree desired, the molten process cheese product is withdrawn from the cooker, cooled to a proper packaging or filling temperature, and packaged. Packaging may take any one of a number of forms. For example, the product may be packaged in loaves or in jars. Alternatively, the molten process cheese product may be formed into slices by distributing the product upon the surface of a cooled rotating chill roll in the form of a thin layer which solidifies into a sheet or ribbon, removing the sheet or ribbon from the chilled surface of the roll, and cutting it into individual slices.
Considerable research has been expended to understand the physical and chemical processes which occur during the manufacture of process cheese-type products. In order for process cheese-type products to have the correct texture, flavor, and meltability attributes, they must undergo a specific temperature and shear history. A certain time at elevated temperatures is required to cook the cheese curd into a continuous phase and to eliminate or minimize the presence of uncooked cheese or "curd specks". In addition to the elevated temperature, a certain degree of agitation at specific shear values is required to facilitate the melting of the cheese and to emulsify the fat in the system.
In one known prior art process for the manufacture of a process cheese-type product, the formulation is subjected to shear in a range of between 10 and 1,000 sec.sup.-1, and the formulation has an absolute viscosity of between about 20 and 50 poise when subjected to shear within this range.
If the shear is excessive, the fat may become over-emulsified, causing defective texture, i.e., the process cheese-type product may become too firm and rubbery and may have restricted melting ability in applications such as hot sandwiches. Hence, the amount of time the process cheese-type product experiences given conditions of shear and temperature may be critical to the finished product attributes.
In the past, continuous processes have been proposed as alternatives to batch processes for manufacture of process cheese-type products. However, one of the problems associated with continuous manufacturing processes is that, where a particular line or apparatus is intended for use in producing more than one type of product, it is generally difficult to provide for a smooth and clean transition from one product to the next, without extensive intermixing of the first product and the second product during the transition period. This is due in part to the fact that plug flow is difficult or impossible to maintain in the context of continuous production of process cheese-type products.
Accordingly, it is a general object of the invention to provide an improved method and apparatus for manufacturing viscous food products.
It is a further object of the invention to provide an improved method and apparatus for manufacturing process cheese-type products which enables a transition to be made from manufacture of a first process cheese-type product to manufacture of a second process cheese-type product without excessive intermixing of the first and second product in the continuous manufacturing operation.
It is a further object of the invention to provide such apparatus for the continuous manufacture of process cheese-type products wherein the apparatus may be cleaned in place, i.e., cleaned without disassembly.
It is a further object of the invention to provide a continuous method for the manufacture of process cheese-type products wherein the shear, temperature, and residence time are controlled to provide a process cheese-type product having the correct texture, flavor, and meltability attributes.
Another object of the present invention is to provide a method and apparatus for the continuous in-line cooking of a process cheese-type formulation, particularly by the direct introduction of steam into a flowing stream of the raw materials used to provide the process cheese-type product.
These and other objects of the invention will become apparent from the following detailed description and the attached drawings.